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Recently, emerging oscillation issues have been caused by subsynchronous interaction (SSI) between voltage source converters (VSCs) based renewable generators and ac/dc grids. To investigate the dynamics of SSI, several impedance models (IMs) of VSC have been developed in previous studies. They are established in different reference frames and consider the entire or just parts of the control systems. However, it is still an unanswered question whether or not the existing IMs are suitable for SSI analysis and to what extent the IMs with different modeling details affect subsynchronous dynamics. To fill in this gap, this paper, first, reviews the existing IMs of VSC and clarifies their relationship. Then, their frequency characteristics are examined so as to select the proper models for the studies of sub-/supersynchronous and high-frequency dynamics. Next, both zero-pole and IM-based SSI analyses are carried out on a simplified system and a real-world wind-farm system that suffered actual SSI incidents. The results indicate that the outer-loop control has a significant impact on the subsynchronous dynamics, and only those IMs taking full-scale controls and frequency-coupling effects into account can reflect SSI accurately. Finally, both modal analysis and electromagnetic transient simulations have been conducted to verify the theoretical results. [ABSTRACT FROM AUTHOR]

The interactions between permanent magnet synchronous generators (PMSGs) and weak grids may cause negatively damped or growing subsynchronous oscillation (SSO). The growing-SSO would evolve into a sustained-SSO when it is predominantly governed by the nonlinearities in the PMSG's converter control, such as current limiter. Most of the existing frequency-domain modeling and stability analysis methods are in the small-signal sense and thus unable to consider the impact of such nonlinearities. Besides, these methods can hardly give vital information to characterize the sustained-SSO, for instance, the magnitude. This paper presents a detailed characteristic analysis of the sustained-SSO while accounting for the effect of current limiter nonlinearity. First, a single-input-single-output (SISO) frequency-domain model of the PMSG is developed. This SISO model comprises the transfer functions of outer-/inner-loop controls and phase-locked loop (PLL). The nonlinear part, i.e., current limiter, is incorporated into the model using the describing function (DF) method. Then, the equivalent transformation on the system's closed-loop characteristic equation is used to separate the SISO model into independent linear and nonlinear parts. This separation enables the use of the DF-based Nyquist criterion for evaluating the system stability and determining oscillation attributes, such as the magnitude and frequency. Both the analytic results and time-domain simulations reveal that the current limiter has a significant impact on the characteristics of the sustained-SSO. [ABSTRACT FROM AUTHOR]